The present invention relates generally to integrated circuit chip package technology, and more particularly to a unique manufacturing methodology for a micro lead frame (MLF) package adapted to improve the longevity of and feed rate for the cutting blades used in a saw singulation process employed during the manufacture of the chip package, and to eliminate the quality reducing sawing burrs which are normally created as a result of the saw singulation process.
Integrated circuit dies are conventionally enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit die and an underlying substrate such as a printed circuit board (PCB). The elements of such a package include a metal lead frame, an integrated circuit die, bonding material to attach the integrated circuit die to the lead frame, bond wires which electrically connect pads on the integrated circuit die to individual leads of the lead frame, and a hard plastic encapsulant material which covers the other components and forms the exterior of the package.
The lead frame is the central supporting structure of such a package. A portion of the lead frame is internal to the package, i.e., completely surrounded by the plastic encapsulant. Portions of the leads of the lead frame extend externally from the package or are partially exposed within the encapsulant material for use in electrically connecting the chip package to another component. In certain chip packages, a portion of the die pad of the lead frame also remains exposed within the exterior of the package for use as a heat sink.
For purposes of high-volume, low-cost production of chip packages, a current industry practice is to etch or stamp a thin sheet of metal material to form a panel or strip which defines multiple lead frames. A single strip may be formed to include multiple arrays, with each such array including a multiplicity of lead frames in a particular pattern. In a typical chip package manufacturing process, the integrated circuit dies are mounted and wire bonded to respective ones of the lead frames, with the encapsulant material then being applied to the strip so as to encapsulate the integrated circuit dies, bond wires, and portions of each of the lead frames in the above-described manner. The hardening of the encapsulant material facilitates the formation of a mold cap upon the lead frames.
Upon the hardening of the encapsulant material, the lead frames within the strip are cut apart or singulated for purposes of producing the individual chip packages. Such singulation is typically accomplished via a saw singulation process. In this process, a saw blade is advanced along “saw streets” which extend in prescribed patterns between the lead frames as required to facilitate the separation of the lead frames from each other in the required manner. The advancement of the saw blade along the saw streets concurrently cuts the molded plastic mold cap, thus facilitating the formation of a molded plastic package body upon each of the separated lead frames.
One of the drawbacks associated with the saw singulation process used in relation to the manufacture of chip packages is that the saw blade used in the saw singulation process cuts through copper (i.e., the metal material typically used to fabricate the strip) one hundred percent of the time. As will be recognized, this level of sawing through copper as occurs as a result of the configuration of the strip often results in the premature wear of the costly saw singulation blades. Another drawback of the saw singulation process is that the same also typically results in the burring of the leads of the separated lead frames. Saw generated burrs at the seating plan of each lead in the lead frame adversely affect solder mounting and joint reliability. In current chip package fabrication methodologies, lead burrs are controlled by limiting the feed rate of the saw blade along the saw streets and by using specifically developed, high cost saw blades. However, as will be recognized, the use of the high cost saw blades is undesirable due to the resultant increase in production costs, with the reduced feed rates needed to control burring adversely affecting production speed, and thus efficiency.
The present invention addresses these drawbacks by providing a unique manufacturing methodology for a chip package wherein the singulation process employed in the manufacturing methodology resolves the issues of slow sawing speed, blade wear and sawing burrs on the leads. In this regard, the singulation process is accomplished in the present invention in a two-step process wherein the saw blade is used to cut only the plastic mold cap applied to the lead frames, with an etching technique being employed to facilitate the separation of the copper lead frames from each other. Since the saw blade is sawing only through the molded plastic of the mold cap, the sawing speed can be set to six inches per second or greater, which represents a substantial increase of the sawing speed of about 0.5 inches per second which is used when the saw blade concurrently cuts through the mold cap and lead frame strip. The sawing of only the mold cap further reduces blade wear of the saw blade, with the etching process used to separate the lead frames from each other leaving no burr on the leads of each lead frame. Thus, the present methodology provides significant cost benefits, and eliminates many of the drawbacks of current saw singulation processes which are often deemed to represent a significant bottleneck in chip package manufacturing methodologies. These, and other advantages of the present invention, will be discussed in more detail below.